Energy vs. Safety - Trade-offs and Risks
The popular media promote a plethora of energy-reduction ideas for consumers (light bulbs, thermostats, screen savers, weather stripping, etc.) and precious few for industry. Since industry consumes 33% of the natural gas, 25% of the renewable energy, 23% of the petroleum and 8% of the coal produced in the U.S. (2011 statistics), this absence might appear to be a major lapse in editorial judgment by the pop-culture pundits.
On the contrary, energy reduction in the industrial sector, while rich with money-saving potential, usually involves a higher level of nuance and narrower applicability than the common denominators typically understood by mass media and the public. In industry, the bulk of energy goes into processes – many of which are chemically and mechanically complex and involve safeguards that must not be disregarded. Below are some examples of how higher levels of risk may accompany energy-saving process changes.
Exhaust-Gas Heat Recovery
Adding a recuperative or regenerative heat exchanger to a combustion process can save energy by preheating the combustion air and simultaneously reducing the temperature of wasted exhaust gases. However, there are risks associated with these process changes in some systems, including burner instabilities due to excessive turndown or disproportionately high temperatures if firing rate can’t be turned down safely.
Exhaust-Gas Flow Reduction
Flue-gas recirculation (FGR) and oxygen-enriched combustion are two contrasting methods for reducing waste of hot gas exhausted to atmosphere. Oxygen-enriched combustion reduces the nitrogen content in the exhaust stream and thereby eliminates the waste of a significant portion of the hot gas stream. In order to transfer the same amount of heat to the load, however, the resulting higher flame temperatures are capable of damaging the burner or furnace. Conversely, FGR is capable of delivering the same amount of heat to a load with reduced exhaust-gas waste and without excessive flame temperatures. Unfortunately, FGR reduces oxygen in the furnace and may cause flame instabilities, nuisance shutdowns or worse.
Better Furnace Insulation
Reducing furnace exterior temperatures by installing better insulation can cut heat losses, which saves fuel year-round and reduces worker discomfort in summer. Under some operating conditions (e.g., oxygen-deficient atmosphere), however, dropping the exterior wall temperature below a critical point can cause hydrocarbons to diffuse and deposit into porous insulating materials and create subsequent problems such as fire or environmental contamination. Alternatively, if acid gases are present in the furnace, these may diffuse through insulating materials and condense on a cold wall, causing corrosion and deterioration of the furnace shell.
High-temperature thermal storage is becoming more commonplace, and molten salt is proving to be a popular storage medium. For metallurgical operations with molten-salt quench baths, a solar concentrating system can be employed to heat and melt the salt, which can then be stored overnight in a well-insulated vessel. However, solar mirrors are not hazard-free devices. Mirrors that focus sunlight can cause skin or eye injuries if they malfunction when workers are in vulnerable locations.
Replacement of traditional fossil fuels with biofuels or waste-to-energy fuel blends may save money and reduce carbon emissions, but burners and other combustion-system components likely need replacement or redesign to safely burn non-traditional fuels. Secondary consequences (e.g., plugging, fouling) related to ash or slag accumulation may also result from the combustion of bio-solids. Storage and handling of combustible solids may lead to unintended accumulations of combustible dust and the explosion hazards associated therewith.
The bottom line: When implementing a new energy-saving idea, seek assistance from a trusted member of the technical staff or outside specialist if the changes could affect process safety. Perform appropriate safety reviews and obtain engineering change approvals if system functionality will be altered in any significant way. IH
This month’s column is my last as a regular contributor to Industrial Heating magazine. I have been writing about safety and environmental issues in this space since January 2008, and it has been a pleasure to have researched and opined on a wide variety of interesting and practical topics. I want express my gratitude to IH and BNP for providing me the opportunity to do something I love – write about thermal technologies. And to IH readers, I send a strong wish for your workplaces to be enriched with excellence as you practice safety for your colleagues and care for the environment.